kernel/linux: fix using custom location

Currently, extract and patch are skipped as thus:
- using a custom directory of pre-installed headers
- a correctly named directory already exists

Otherwise, extract and patch are done.

The current second condition is wrong, because it allows the following
sequence to happen:
- a non-custom kernel is used
- a previous build only partially extracted the non-custom sources
- that p[revious build broke during extraction (eg. incomplete tarball...)
- a subsequent build will find a properly named directory, and will
thus skip extract and patch, which is wrong

Fix that by following the conditions in this table:

Type | Extract | Patch
----------------------+---------+-------
Pre-installed headers | N | N
custom directory | N | N
custom tarball | Y | N
mainstream tarball | Y | Y

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: David Holsgrove <david.holsgrove@xilinx.com>

This is a somewhat ordered TODO list:

Recurring tasks:

- update versions for every tools...

- update and/or upgrade all samples

Non-recurring tasks:

- update newlib (for enhanced bare metal)

  - confirm existing implementation on targets other than AVR32

  - try to make it generic, will help for uClibc++

- multilib

- Cygwin target (needs newlib)

  - so we can build toolchains that generate code for Windows/Cygwin

- Cygwin host

  - so we can build toolchains that run natively under Windows/Cygwin

- mingw32 target

  - so we can build toolchains that generate code for Windows/Win32

- mingw32 host

  - so we can build toolchains that run natively under Windows/Win32

    (without requiring Cygwin)

- uClibc++

  - see newlib, above

- uClinux

  - check whether we can use newlib under uClinux (?)